In Microsoft Forms, open the form or quiz with the collaborate link.
Select ... and then Collaborate or Duplicate.
Select the trash can button next to the text box that displays the form link.
Select Remove link to permanently remove the collaborate link. (Select Cancel if you change your mind and don't want to remove the collaborate link.)
Once you've removed the collaborate link, it's unusable for anyone that tries to click or tap on it. You can't recreate the same collaborate link and will need to get a new, unique one to share with others.
